vtbackup: Allow initial backup as long as no tablets are serving.
vtbackup: Allow initial backup as long as no tablets are serving.

Requiring that no tablets are running at all was too strict. It
prevented using the new -wait_for_backup_interval flag to start up
tablets concurrently while the initial backup is going.

Signed-off-by: Anthony Yeh <1cdc736ba6fd89dd9b69239aea62ddb9fd1482b1@planetscale.com>

mahak/vitess,davygeek/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,davygeek/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,davygeek/vitess,tinyspeck/vitess,davygeek/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,davygeek/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,davygeek/vitess,vitessio/vitess,davygeek/vitess,tinyspeck/vitess